In most cases plastic bottles are made of PET, which forms a single or multi-layer bottle wall. In the bottom area, the bottles must have a safe stand and have ability to withstand internal pressure. In this context, "stability" denotes stability during normal use. Another type of stability mentioned above is thermostability when hot-washing the bottle and thus the pre-condition for a reusability of the bottle. The bottom area must have an additional feature, i.e. it must be "inspectable", a term which experts in this technical field use to characterize an unobstructed view through the bottom area in direction of the bottle axis. Inspection of the bottom must not be affected by shadows and other optical refraction, because this would make visual inspection of the empty bottle by a camera or the eye impossible.